A liquid crystal display device, which is a type of display devices, is equipped with a backlight unit because a liquid crystal display panel provided therein for displaying an image does not emit light. The backlight unit is mounted on a rear surface (a surface opposite to a display surface of the liquid crystal display panel) of the liquid crystal display panel, and by illuminating the liquid crystal display panel with the backlight unit, the display operation is performed.
The backlight unit for the liquid crystal display device is broadly categorized into two types: the direct lighting type and the edge lighting type.
In the direct lighting type backlight unit, a light source is provided immediately below the liquid crystal display panel. Such a direct lighting type backlight unit is suitable for illuminating a large area with high output power, and is therefore mainly used for a large liquid crystal display device.
On the other hand, in the edge lighting type backlight unit, a light guide plate is provided immediately below the liquid crystal display panel, and a light source is arranged so as to face an end surface of the light guide plate on a prescribed side. Typically, a light diffusion sheet for diffusing light and a lens sheet for collecting light are disposed on an upper surface (surface facing the liquid crystal display panel) of the light guide plate, and these light diffusion sheet and lens sheet are fixed in position by an adhesive tape. Such an edge lighting type backlight unit has an advantage of providing a thinner profile than that of the direct lighting type backlight unit.
The illumination operation of the edge lighting type backlight unit is performed as follows. When light is generated by the light source, the light from the light source enters the light guide plate through the end surface of the light guide plate on the prescribed side. The light that has entered the light guide plate is emitted through the upper surface of the light guide plate, and by passing through the light diffusion sheet and the lens sheet, the light illuminates the liquid crystal display panel (see Patent Document 1, for example).
In recent years, a thinner liquid crystal display device has been sought after, creating a stronger demand for the backlight unit having even thinner components. As for the lens sheet, which is a component of the backlight unit, for example, the thickness thereof needs to be reduced by about half (from about 100 μm to about 50 μm, for example).